During pregnancy in the human and many other mammals, part of the chorion invades the uterine mucosa and becomes intimately associated with the maternal vascular supply for the purposes of physiological exchange. At present, the factors which control trophoblastic growth and invasion during development of the chorioallantoic placenta in these species are poorly understood. The proposed study will examine the hypothesis that extracellular materials associated with endometrial blood vessels and possibly produced by their endothelial cells may have a major influence upon the pattern of trophoblastic growth. A wild mammal, the free-tailed bat, will be used as the animal model for these studies because it possesses the following unique combination of reproductive characteristics: it develops a discoidal, hemochorial, chorioallantoic placenta like the human; the chorioallantoic placenta always develops at the same location in the uterus; maternal blood vessels at that site differ from those elsewhere in the uterus even prior to conception; one of the specializations exhibited by these vessels (endothelial hypertrophy) is also seen in the uterine vessels of many other mammals, including some primates, during placental development; and during pregnancy in this bat trophoblast invades preferentially towards those specialized vessels. The sequence of major events in placental morphogenesis in the bat will first be worked out by the histological examination of reproductive tracts removed at timed intervals after lab matings. Light and electron microscopic studies will then be pursued to determine how the vessels and endometrial stroma at the placental site differ morphologically and cytochemically from those in adjacent areas of the uterus which are less aggressively invaded by trophoblast. Since extracellular glycoproteins and glycosaminoglycans may in particular exert major influence upon trophoblastic growth, their abundance and distribution will be followed until the definitive hemochorial placenta has been established. Finally, autoradiographic studies will be carried out in an effort to determine which cells are secreting components of the prominent perivascular matrix at the placental site and whether these labelled maternal components become incorporated into the definitive placenta. This work should help to establish the role of maternal blood vessels in placental morphogenesis.